Method for modifying a bit sequence in an ARQ retransmission, receiver and transmitter therefor

ABSTRACT

An ARQ transmission method in a communication system, wherein data packets comprising modulation symbols are transmitted based on an automatic repeat request and subsequently combined with previously received data packets. The symbols of the transmitted data packets are modulated In a mapping entity employing at least a first and second signal constellation. The method further comprises the step of obtaining the second signal constellation from the first signal constellation by exchanging a logical bit position and/or inverting a logical bit. The invention further relates to a corresponding transmitter and receiver.

[0001] The present invention relates to a method for modifying a bitsequence in an ARQ retransmission in a communication system. Further,the invention concerns a corresponding receiver and transmitter.

[0002] A common technique in communication systems with unreliable andtime-varying channel conditions is to correct errors based on automaticrepeat request (ARQ) schemes together with a forward error correction(FEC) technique called hybrid ARQ (HARQ). If an error is detected by acommonly used cyclic redundancy check (CRC), the receiver of thecommunication system requests the transmitter to resend the erroneouslyreceived data packets.

[0003] S. Kallel, Analysis of a type II hybrid ARQ scheme with codecombining, IEEE Transactions on Communications, Vol. 38, No. 8, August1990 and S. Kallel, R. Link, S. Bakhtiyari, Throughput performance ofMemory ARQ schemes, IEEE Transactions on Vehicular Technology, Vol. 48,No. 3, May 1999 define three different types of ARQ schemes:

[0004] Type I: The erroneous received packets are discarded and a newcopy of the same packet is retransmitted and decoded separately. Thereis no combining of earlier and later received versions of that packet.

[0005] Type II: The erroneous received packets are not discarded, butare combined with additional retransmissions for subsequent decoding.Retransmitted packets sometimes have higher coding rates (coding gain)and are combined at the receiver with the stored soft-information fromprevious transmissions.

[0006] Type III Is the same as Type II with the constraint eachretransmitted packet is now self-decodable. This implies that thetransmitted packet is decodable without the combination with previouspackets. This is useful if some packets are damaged in such a way thatalmost no information is reusable. If all transmissions carry identifieddata, this can be seen as a special case called HARQ Type III with asingle redundancy version.

[0007] Types II and III schemes are obviously more intelligent and showa performance gain with respect to Type I, because they provide theability to reuse information from of previously received erroneouspackets. There exist basically three schemes of reusing the redundancyof previously transmitted packets:

[0008] Soft-Combining

[0009] Code-Combining

[0010] Combination of Soft- and Code-Combining

[0011] Soft-Combining

[0012] Employing soft-combining the retransmission packets carryidentical information compared with the previously received information.In this case the multiple received packets are combined either by asymbol-by-symbol or by a bit-by-bit basis as for example disclosed in D.Chase, Code combining: A maximum-likelihood decoding approach forcombining an arbitrary number of noisy packets, IEEE Trans. Commun.,Vol. COM-33, pp. 385-393, May 1985 or B. A. Harvey and S. Wicker, PacketCombining Systems based on the Viterbi Decoder, IEEE Transactions onCommunications, Vol. 42, No. 2/3/4, April 1994. By combining thissoft-decision values from all received packets the reliabilities of thetransmitted bits will increase linearly with the number and power ofreceived packets. From a decoder point of view the same FEC scheme (withconstant code rate) will be employed over all transmissions. Hence, thedecoder does not need to know how many retransmissions have beenperformed, since it sees only the combined soft-decision values. In thisscheme all transmitted packets will have to carry the same number ofsymbols.

[0013] Code-Combining

[0014] Code-combining concatenates the received packets in order togenerate a new code word (decreasing code rate with increasing number oftransmission). Hence, the decoder has to be aware of the FEC scheme toapply at each retransmission instant. Code-combining offers a higherflexibility with respect to soft-combining, since the length of theretransmitted packets can be altered to adapt to channel conditions.However, this requires more signaling data to be transmitted withrespect to soft-combining.

[0015] Combination of Soft- and Code-Combining

[0016] In case the retransmitted packets carry some symbols identical topreviously transmitted symbols and some code-symbols different fromthese, the identical code-symbols are combined using soft-combing asdescribed in the section titled “Soft Combining” while the remainingcode-symbols will be combined using code-combining. Here, the signalingrequirements will be similar to code-combining.

[0017] As it has been shown in M. P. Schmitt Hybrid ARQ Scheme employingTCM and Packet Combining, Electronics Letters Vol. 34, No. 18, September1998 that HARQ performance for Trellis Coded Modulation (TCM) can beenhanced by rearranging the symbol constellation for theretransmissions. There, the performance gain results from the maximizingthe Euclidean distances between the mapped symbols over theretransmissions, because the rearrangement has been performed on asymbol basis.

[0018] Considering high-order modulation schemes (with modulationsymbols carrying more than two bits) the combining methods employingsoft-combining have a major drawback: The bit reliabilities withinsoft-combined symbols will be in a constant ratio over allretransmissions, i.e. bits which have been less reliable from previousreceived transmissions will still be less reliable after having receivedfurther transmissions and, analogous, bits which have been more reliablefrom previous received transmissions will still be more reliable afterhaving received further transmissions.

[0019] The varying bit reliabilities evolve from the constraint oftwo-dimensional signal constellation mapping, where modulation schemescarrying more than 2 bits per symbol cannot have the same meanreliabilities for all bits under the assumption that all symbols aretransmitted equally likely. The term mean reliabilities is consequentlymeant as the reliability of a particular bit over all symbols of asignal constellation.

[0020] Employing a signal constellation for a 16 QAM modulation schemeaccording to FIG. 1 showing a Gray encoded signal constellation with a:given bit-mapping order i₁q₁i₂q₂, the bits mapped onto the symbolsdiffer from each other in mean reliability in the first transmission ofthe packet. In more detail, bits i₁ and q₁ have a high mean reliability,as these bits are mapped to half spaces of the signal constellationdiagram with the consequences that their reliability is independent fromthe fact of whether the bit transmits a one or a zero.

[0021] In contrast thereto, bits i₂ and q₂ have a low mean reliability,as their reliability depends on the fact of whether they transmit a oneor a zero. For example, for bit i₂, ones are mapped to outer columns,whereas zeros are mapped to inner columns. Similarly, for bit q₂, onesare mapped to outer rows, whereas zeros are mapped to inner rows.

[0022] For the second and each further retransmissions the bitreliabilities will stay in a constant ratio to each other, which isdefined by the signal constellation employed in the first transmission,i.e. bits i₁ and q₁ will always have a higher mean reliability than bitsi₂ and q₂ after any number of retransmissions.

[0023] In, co-pending PCT/EP01/01982a method has been suggested that inorder to enhance the decoder performance, it would be quite beneficialto have equal or near to equal mean bit reliabilities after eachreceived transmission of a packet. Hence, the bit reliabilities aretailored over the retransmissions in a way that the mean bitreliabilities get averaged out. This is achieved by choosing apredetermined first and at least second signal constellation for thetransmissions, such that the combined mean bit reliabilities for therespective bits of all transmissions are nearly equal.

[0024] Hence, the signal constellation rearrangement results in achanged bit mapping, wherein the Euclidean distances between themodulation symbols can be altered from retransmission to retransmissiondue to the movement of the constellation points. As a result, the meanbit reliabilities can be manipulated in a desired manner and averagedout to increase the performance the FEC decoder at the receiver.

[0025] In the solution proposed above, the benefits of the constellationrearrangement are realized through a parameterized bit-to-symbol mappingentity. For complexity or efficient implementational reasons, it may beadvantageous for a communication system to have a non-parameterizedstandard mapping entity.

[0026] Consequently, the object of the present invention resides inproviding an ARQ transmission method, a transmitter and a receiver withan improved error correction performance without a parameterizedbit-to-symbol mapping entity.

[0027] This object is solved by a method comprising the steps as definedin claim 1. Further, the object is solved by a transmitter and receiveras defined by the independent claims.

[0028] The idea underlying the present invention is to modify the inputbit sequence prior to entry of same into the mapping entity. Thismodification of the signal constellation can be achieved by using aninterleaver and a logical bit inverter, which invert and/or exchange thepositions of the signal constellation bits dependent on theretransmission number parameter m. Hence, the beneficial effects of aconstellation rearrangement are achieved without the need for aparameterized bit to symbol mapping entity. As a result, the sequencewhich is output after processing by the interleaver the logical bitinverter and a non-parameterized standard mapping entity isindistinguishable from the output of a parameterized bit to symbolmapping entity employing various constellation rearrangement schemes.

[0029] For a better understanding of the invention, preferredembodiments will be described in the following with reference to theaccompanying drawings.

[0030]FIG. 1 is an exemplary signal constellation for illustrating a 16QAM modulation scheme with Gray encoded bit symbols,

[0031]FIG. 2 shows four examples for signal constellations for a 16 QAMmodulation scheme with Gray encoded bit symbols, and

[0032]FIG. 3 is an exemplary embodiment of a communication system inwhich the method underlying the invention is employed.

[0033] In the following the concept of a Log-Likelihood-Ratio (LLR) willbe described as a metric for the bit reliabilities. First the straightforward calculation of the bit LLRs within the mapped symbols for asingle transmission will be shown. Then the LLR calculation will beextended to the multiple transmission case.

[0034] Single Transmission

[0035] The mean LLR of the i-th bit b_(n) ^(i) under the constraint thatsymbol s_(n) has been transmitted for a transmission over a channel withadditive white gaussian noise (AWGN) and equally likely symbols yields$\begin{matrix}{{{{LLR}_{b_{n}^{i}|r_{n}}\left( r_{n} \right)} = {{\log \left\lbrack {\sum\limits_{({{m|b_{m}^{i}} = b_{n}^{i}})}^{{- \frac{E_{S}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack} - {\log \left\lbrack {\sum\limits_{({m|{b_{m}^{i} \neq b_{n}^{i}}})}^{{- \frac{E_{S}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack}}},} & (1)\end{matrix}$

[0036] where r_(n)=s_(m), denotes the mean received symbol under theconstraint the symbol s_(n) has been transmitted (AWGN case), d_(n,m) ²denotes the square of the Euclidean distance between the received symbolr_(n) and the symbol s_(m), and E_(S)/N_(o) denotes the observedsignal-to-noise ratio.

[0037] It can be seen from Equation (1) that the LLR depends on thesignal-to-noise ratio E_(S)/N_(o) and the Euclidean distances d_(n,m)between the signal constellation points.

[0038] Multiple Transmissions

[0039] Considering multiple transmissions the mean LLR after the k-thtransmission of the i-th bit b_(n) ^(i) under the constraint thatsymbols S_(n) ^((j)) have been transmitted over independent AWGNchannels and equally likely symbols yields $\begin{matrix}{{{{LLR}_{b_{n}^{i}|{\bigcap_{j = 1}^{k}r_{n}^{(j)}}}\left( {r_{n}^{(1)},r_{n}^{(2)},\quad \ldots \quad,r_{n}^{(k)}} \right)} = {{\log \left\lbrack {\sum\limits_{({{m|b_{m}^{i}} = b_{n}^{i}})}^{- {\sum\limits_{j = 1}^{k}\quad {{(\frac{E_{S}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(j)})}^{2}}}}} \right\rbrack} - {\log \left\lbrack {\sum\limits_{({m|{b_{m}^{i} \neq b_{n}^{i}}})}^{- {\sum\limits_{j = 1}^{k}\quad {{(\frac{E_{S}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(j)})}^{2}}}}} \right\rbrack}}},} & (2)\end{matrix}$

[0040] where j denotes the j-th transmission ((j-1)-th retransmission).Analogous to the single transmission case the mean LLRs depend on thesignal-to-noise ratios and the Euclidean distances at each transmissiontime.

[0041] If no constellation rearrangement is performed the Euclideandistances d_(n,m) ^((j))=d_(n,m) ⁽¹⁾ are constant for all transmissionsand, hence, the bit reliabilities (LLRs) after k transmissions will bedefined by the observed signal-to-noise ratio at each transmission timeand the signal constellation points from the first transmission. Forhigher level modulation schemes (more than 2 bits per symbol) thisresults in varying mean LLRs for the bits, which in turn leads todifferent mean bit reliabilities. The differences in mean reliabilitiesremain over all retransmissions and lead to a degradation in decoderperformance.

[0042] In the following, the case of a 16-QAM system will be exemplarilyconsidered resulting in 2 high reliable and 2 low reliable bits, wherefor the low reliable bits the reliability depends on transmitting a oneor a zero (see FIG. 1). Hence, overall there exist 2 levels of meanreliabilities, whereby the second is further subdivided.

[0043] Level 1 (High Reliability, 2 bits): Bit mapping for ones (zeros)separated into the positive (negative) real half space for the i-bitsand the imaginary half space the q-bits. Here, there is no differencewhether the ones are mapped to the positive or to the negative halfspace.

[0044] Level 2 (Low Reliability, 2 bits): Ones (zeros) are mapped toinner (outer) columns for the i-bits or to inner (outer) rows for theq-bits. Since there is a difference for the LLR depending on the mappingto the inner (outer) columns and rows, Level 2 is further classified:

[0045] Level 2a: Mapping of i_(n) to inner columns and q_(n) to innerrows respectively.

[0046] Level 2b: Inverted mapping of Level 2a: Mapping of i_(n) to outercolumns and q_(n) to outer rows respectively.

[0047] To ensure an optimal averaging process over the transmissions forall bits the levels of reliabilities have to be altered.

[0048] It has to be considered that the bit-mapping order is open priorinitial transmission, but has to remain through retransmissions, e.g.pit-mapping for initial transmission: i₁q₁i₂q₂

bit-mapping all retransmissions: i₁q₁i₂q₂.

[0049] Some examples for possible constellations are shown in FIG. 2.The resulting bit reliabilities according to FIG. 2 are given in Table1. TABLE 1 Constellation bit i₁ bit q₁ bit i₂ bit q₂ 1 High High Low LowReliability Reliability Reliability Reliability (Level 1)  (Level 1) (Level 2b) (Level 2b) 2 Low Low High High Reliability ReliabilityReliability Reliability (Level 2a) (Level 2a) (Level 1)  (Level 1)  3Low Low High High Reliability Reliability Reliability Reliability (Level2b) (Level 2b) (Level 1)  (Level 1)  4 High High Low Low ReliabilityReliability Reliability Reliability (Level 1)  (Level 1)  (Level 2a)(Level 2a)

[0050] In the following, it is assumed that m denotes the retransmissionnumber parameter, with m=0 denoting the first transmission of a packetin the ARQ context. Further let b denote the number of bits that form asymbol in the mapping entity. Typically, b can be any integer number,where the most often used values for communication systems are aninteger power of 2.

[0051] Without loss of generality it can be further assumed that thenumber of bits n that are used as input to the interleaving process isdividable by b, i.e. n is an integer multiple of b. Those skilled in theart will perceive that if this should not be the case, then the sequenceof input bits can be easily appended by dummy bits until the abovecondition is met.

[0052] As described above, for a given modulation, several reliabilitylevels can be identified. The interleaving process should thus averageout the reliabilities of the b bits over the retransmissions such thatall b bits are in average equally reliable. This means that theinterleaver has to change the positions of the b bits within a symbol(also termed “wrapping” in the art) such that each of the original bitsis mapped as often to all reliability levels as every other of the bbits. This means that the interleaving is an intra-symbol bitinterleaving process.

[0053] Additionally, there can be several bit positions for which thereliabilities depend on the logical bit value (low or high). When a bitis mapped for the non-first time on such a position, this bit shouldalso be logically inverted.

[0054] With these rules, patterns can be constructed which determine theinterleaver and inverter process for a transmission number m.

[0055] In theory, the perfect averaging out of the reliabilities mightbe possible only after an infinite or very high number ofretransmissions. In these cases, there might thus be severalalternatives which differ in the sequence of interleaver or inverterpatterns. Which of these alternatives is chosen is left open to thechoice of the system designer, since it will mean no difference inperformance.

[0056] If the signal constellation as in FIG. 1 is to be kept, in orderto get constellation 2 from constellation 1 in FIG. 2, the followingprocesses have to be executed, where the order is irrelevant:

[0057] exchange positions of original bits i₁ and i₂

[0058] exchange positions of original bits q₁ and q₂

[0059] logical bit inversion of original bits i₁ and q₁

[0060] Alternatively, those bits that end in positions 1 and 2 can alsobe inverted.

[0061] An example dependent on the transmission number is given in thefollowing table, where the bits always refer to the first transmission,and a long dash above a character denotes logical bit inversion of thatbit: TABLE 2 Constellation number Interleaver and Inverter functionality1 i₁q₁i₂q₂ 2 i₂q₂{overscore (i)}₁{overscore (q)}₁ or {overscore(i)}₂{overscore (q)}₂{overscore (i)}₁{overscore (q)}₁ 3 {overscore(i)}₂{overscore (q)}₂i₁q₁ or i₂q₂i₁q₁ 4 i₁q₁{overscore (i)}₂{overscore(q)}₂ or {overscore (i)}₁{overscore (q)}₁{overscore (i)}₂{overscore(q)}₂

[0062] The first given examples in each row of table 2 correspond to theconstellations given in FIG. 2. As readily apparent from table 2, signalconstellation 2 is obtained from constellation 1 by exchanging(swapping) the positions of bits i₁ and i₂ as well as that of bits q₁and q₂ and by inverting either bit pair i₁, q₁ or all bits. Similarly,signal constellation 3 is obtained from constellation 1 by exchangingpositions of bits i₁ and i₂ as well as that of bits q, and q₂ with eachother respectively and by inverting bit pair i₂, q₂ in one alternative.In the other alternative, only the bit positions are exchanged and noinversion is necessary. Finally, signal constellation 4 is obtained fromconstellation 1 by inverting either bit pair i₂, q₂ or all bits of thesymbol without exchanging any bit position.

[0063] From this, it can be chosen between different strategies fortransmission numbers (non-exhaustive): TABLE 3 TransmissionConstellation Constellation Constellation Constellation ConstellationConstellation Number Number Number Number Number Number Number 1 1 1 1 11 1 2 2 2 3 4 4 3 3 3 4 2 2 3 4 4 4 3 4 3 2 2

[0064]FIG. 3 shows an exemplary embodiment of a communication system inwhich the method underlying the invention is employed.

[0065] At the transmitter 100, a bit sequence is obtained from a forwarderror correction (FEC) encoder (not shown) and subsequently input intoan interleaver 110 and a logical bit inverter 120. The interleaver 110and logical bit inverter are each dependent on the retransmission numberparameter m and modify the input bit sequence. Subsequently, the bitsequence is input into the mapper/modulator 130 being anon-parameterized standard mapping entity. The mapper typically uses oneof the signal constellations shown in FIG. 2 and maps the b bits onto asymbol which is transmitted over the communication channel 200. Thecommunication channel is typically a radio communication channelexperiencing unreliable and time-varying channel conditions.

[0066] The interleaving/inverting patterns is either stored at both, thetransmitter and the receiver or stored at the transmitter and signalledto the receiver.

[0067] At the receiver 300, the complex symbols are first input into ademapper/demodulator 330 which demodulates the received symbols into acorresponding bit domain sequence (e.g. sequence of LLRs). This sequenceis then input into a logical inverter 320 and subsequently into ade-interleaver 310 from which the obtained bit domain sequence isoutput.

[0068] The interleaver and de-interleaver operate in accordance with thewell known technique of interleaving/deinterleaving by applying adetermined, pseudo-random or random permutation of the input bit orsymbol sequences, i.e. exchange (swap) the positions of the bits orsymbols within a sequence. In the above described embodiment, theinterleaver is a intra-symbol bit interleaver which changes the positionof the bits that form a symbol in the mapping entity.

[0069] The logical bit inverter operates in accordance with a well knowntechnique of inverting the logical value of a bit, i.e. turns a logicallow to a logical high value and vice versa. In a practical realizationfor a receiver working with log likelihood ratios, this invertingoperation is equivalent to a sign inversion of the log likelihood ratio.

[0070] If a retransmission is launched by an automatic repeat requestissued by an error detector (not shown) with the result that anidentical data packet is transmitted from the transmitter 100, in thede-mapper/demodulator 330, the previously received erroneous datapackets are soft-combined with the retransmitted data packets. Due tothe modification of the bit sequence by the interleaver and the logicalbit inverter, the mean bit reliabilities are averaged out resulting inan increased performance in the receiver.

[0071] Although the method described above has been described usingGray-encoded signals and a QAM modulation scheme, it is clear to askilled person that other suitable encoding and modulation schemes canbe equally used in obtaining the benefits of the invention.

1-14. (Canceled).
 15. A transmission apparatus comprising: atransmission section that (i) transmits a first data arranged in a first16 QAM constellation pattern in a first transmission, and (ii)retransmits a second data arranged in a second 16 QAM constellationpattern in a retransmission, wherein said second 16 QAM constellationpattern is a rearrangement of said first 16 QAM constellation pattern,and wherein, in said first transmission, a bit sequence (i₁q₁i₂q₂) isused as said first 16 QAM constellation pattern, and, in saidretransmission, said bit sequence is modified, to obtain said second 16QAM constellation pattern, by exchanging positions of i₁ and i₂, byexchanging positions of q₁ and q₂, and by logically inverting i₁ and q₁.16. A transmission apparatus comprising: a transmission section that (i)transmits a first data arranged in a first 16 QAM constellation patternin a first transmission, and (ii) retransmits a second data arranged ina second 16 QAM constellation pattern in a retransmission, wherein saidsecond 16 QAM constellation pattern is a rearrangement of said first 16QAM constellation pattern, and wherein, in said first transmission, abit sequence (i₁q₁i₂q₂) is used as said first 16 QAM constellationpattern, and, in said retransmission, said bit sequence is modified, toobtain said second 16 QAM constellation pattern, by exchanging positionsof i₁ and i₂, by exchanging positions of q₁ and q₂, and by logicallyinverting i₂ and q₂.
 17. A transmission apparatus comprising: atransmission section that (i) transmits a first data arranged in a first16 QAM constellation pattern in a first transmission, and (ii)retransmits a second data arranged in a second 16 QAM constellationpattern in a retransmission, wherein said second 16 QAM constellationpattern is a rearrangement of said first 16 QAM constellation pattern,and wherein, in said first transmission, a bit sequence (i₁q₁i₂q₂) isused as said first 16 QAM constellation pattern, and, in saidretransmission, said bit sequence is modified, to obtain said second 16QAM constellation pattern, by logically inverting i₂ and q₂.
 18. Thetransmission apparatus according to claim 15, 16 or 17, wherein saidrearrangement is performed such that a mean bit reliability for each bitmapped onto a symbol is averaged out through repetition provided by saidretransmission.
 19. The transmission apparatus according to claim 15, 16or 17, wherein the data is Gray encoded.
 20. A base station apparatusequipped with the transmission apparatus according to claim 15, 16 or17.
 21. A communication terminal apparatus equipped with thetransmission apparatus according to claim 15, 16 or
 17. 22. Atransmission method comprising: transmitting a first data arranged in afirst 16 QAM constellation pattern in a first transmission; andretransmitting a second data arranged in a second 16 QAM constellationpattern in a retransmission, wherein: said second 16 QAM constellationpattern is a rearrangement of said first 16 QAM constellation pattern,in said first transmission, a bit sequence (i₁q₁i₂q₂) is used as saidfirst 16 QAM constellation pattern, and in said retransmission, said bitsequence is modified, to obtain said second 16 QAM constellationpattern, by exchanging positions of i₁ and i₂, by exchanging positionsof q₁ and q₂, and by logically inverting i₁ and q₁.
 23. A transmissionmethod comprising: transmitting a first data arranged in a first 16 QAMconstellation pattern in a first transmission; and retransmitting asecond data arranged in a second 16 QAM constellation pattern in aretransmission, wherein: said second 16 QAM constellation pattern is arearrangement of said first 16 QAM constellation pattern, in said firsttransmission, a bit sequence (i₁q₁i₂q₂) is used as said first 16 QAMconstellation pattern, and in said retransmission, said bit sequence ismodified, to obtain said second 16 QAM constellation pattern, byexchanging positions of i₁ and i₂, by exchanging positions of q₁ and q₂,and by logically inverting i₂ and q₂.
 24. A transmission methodcomprising: transmitting a first data arranged in a first 16 QAMconstellation pattern in a first transmission; and retransmitting asecond data arranged in a second 16 QAM constellation pattern in aretransmission, wherein: said second 16 QAM constellation pattern is arearrangement of said first 16 QAM constellation pattern, in said firsttransmission, a bit sequence (i₁q₁i₂q₂) is used as said first 16 QAMconstellation pattern, and in said retransmission, said bit sequence ismodified, to obtain said second 16 QAM constellation pattern, bylogically inverting i₂ and q₂.
 25. A transmission apparatus comprising:a transmission section that (i) transmits a first data mapped based on afirst 16 QAM constellation pattern in a bit sequence (i₁q₁i₂q₂) in afirst transmission, and (ii) retransmits a second data mapped based on asecond 16 QAM constellation pattern in a retransmission, wherein saidsecond 16 QAM constellation pattern is obtained, as an arranged patternof said bit sequence (i₁q₁i₂q₂), by exchanging positions of i₁ and i₂,by exchanging positions of q₁ and q₂, and by logically inverting i₁ andq₁.
 26. A transmission apparatus comprising: a transmission section that(i) transmits a first data mapped based on a first 16 QAM constellationpattern in a bit sequence (i₁q₁i₂q₂) in a first transmission, and (ii)retransmits a second data mapped based on a second 16 QAM constellationpattern in a retransmission, wherein said second 16 QAM constellationpattern is obtained, as an arranged pattern of said bit sequence(i₁q₁i₂q₂), by exchanging positions of i₁ and i₂, by exchangingpositions of q₁ and q₂, and by logically inverting i₂ and q₂.
 27. Atransmission apparatus comprising: a transmission section that (i)transmits a first data mapped based on a first 16 QAM constellationpattern in a bit sequence (i₁q₁i₂q₂) in a first transmission, and (ii)retransmits a second data mapped based on a second 16 QAM constellationpattern in a retransmission, wherein said second 16 QAM constellationpattern is obtained, as an arranged pattern of said bit sequence(i₁q₁i₂q₂), by logically inverting i₂ and q₂.
 28. A communicationterminal apparatus equipped with the transmission apparatus according toclaim 25, 26 or
 27. 29. A base station apparatus equipped with thetransmission apparatus according to claim 25, 26 or
 27. 30. Atransmission method comprising: transmitting a first data mapped basedon a first 16 QAM constellation pattern in a bit sequence (i₁q₁i₂q₂) ina first transmission, and retransmitting a second data mapped based on asecond 16 QAM constellation pattern in a retransmission, wherein: saidsecond 16 QAM constellation pattern is obtained, as an arranged patternof said bit sequence (i₁q₁i₂q₂), by exchanging positions of i₁ and i₂,by exchanging positions of q₁ and q₂, and by logically inverting i₁ andq₁.
 31. A transmission method comprising: transmitting a first datamapped based on a first 16 QAM constellation pattern in a bit sequence(i₁q₁i₂q₂) in a first transmission, and retransmitting a second datamapped based on a second 16 QAM constellation pattern in aretransmission, wherein: said second 16 QAM constellation pattern isobtained, as an arranged pattern of said bit sequence (i₁q₁i₂q₂), byexchanging positions of i₁ and i₂, by exchanging positions of q₁ and q₂,and by logically inverting i₂ and q₂.
 32. A transmission methodcomprising: transmitting a first data mapped based on a first 16 QAMconstellation pattern in a bit sequence (i₁q₁i₂q₂) in a firsttransmission, and retransmitting a second data mapped based on a second16 QAM constellation pattern in a retransmission, wherein: said second16 QAM constellation pattern is obtained, as an arranged pattern of saidbit sequence (i₁q₁i₂q₂), by logically inverting i₂ and q₂.